Police in St Louis have released a graphic video of the moment a young black man was shot dead by officers that raises questions about whether events transpired exactly as initially claimed.

A convenience store owner called 911 on Tuesday when he suspected Powell stole drinks and donuts from his shop, according to a recording of the call. Another woman called to report Powell was acting erratically and had a knife in his pocket.

Two officers in a police SUV responded to the calls, the cell phone video shows. When the officers got out of their vehicle, Powell "pulled out a knife and came at the officers, gripping and holding it high," yelling "Shoot me now, kill me now."

St Louis Police Chief Sam Dotson said Tuesday that both of the officers opened fire on Powell when he came within a three or four feet of them holding a knife "in an overhand grip."

But the newly released cell phone footage undermines the statement. It begins before police arrive on the scene, filmed by a bystander who can be heard laughing at Powell's behaviour.

Whe the police arrive, Powell can be seen approaching the cops, but not coming as close as was reported, with his hands at his side. The officers began shooting within 15 seconds of their arrival, hitting Powell with a barrage of bullets.

The St Louis Metropolitan Police Department released the video and 911 calls, telling St. Louis Public Radio that it plans to act transparently.

The shooting death occurred less than four miles from where Michael Brown was fatally shot by a police officer in Ferguson on Aug. 9, where violent protests have grubbed the suburb.

Dotson defended the officers in a television appearance later Wednesday, acknowledging the discrepancies between his Tuesday account and what the video revealed. Though Powell's hands were down by his sides, Dotson told CNN, he was moving toward the officer with a knife.

"The officers did what I think you or I would do, they protected their life in that situation," Dotson said.

CNN's Don Lemon and Chris Cuomo pushed Dotson on the officers' decision to use lethal force, rather than opting for an alternate form of defense like a Taser.

"Certainly a Taser is an option that's available to the officers, but Tasers aren't 100 percent," Dotson said. "So you've got an individual with a knife who's moving towards you, not listening to any verbal commands, continues, says, 'shoot me now, kill me now.' Tasers aren't 100 percent. if that Taser misses, that [individual] continues on and hurts an officer."